Fuel assembly for a boiling water reactor

ABSTRACT

A fuel assembly for a boiling water reactor comprises a bottom tie plate, a top tie plate, and a first and a second group of vertical fuel rods, each including a column of fuel pellets surrounded by a cladding tube. The second group of fuel rods has a shorter active length than the first group of fuel rods. Both the first and the second group of fuel rods extend between the bottom tie plate and the top tie plate. The second group of fuel rods comprises a fission gas plenum surrounded by a plenum tube arranged above the cladding tube. The main part of the plenum tube has a cross-section area which is smaller than the cross-section area of the cladding tube.

TECHNICAL FIELD

The present invention relates to a fuel assembly for a boiling waterreactor comprising a bottom tie plate, a top tie plate, a first and asecond group of vertical fuel rods, each comprising a column of fuelpellets surrounded by a cladding tube, wherein the second group of fuelrods has a shorter active length than the first group of fuel rods. Theactive length of a fuel rod means the height of the column of fuelpellets.

BACKGROUND OF THE INVENTION

A core in a nuclear reactor comprises a plurality of vertically arrangedfuel assemblies. A fuel assembly comprises one or more fuel bundles. Afuel bundle comprises a plurality of vertical fuel rods arranged betweena bottom tie plate and a top tie plate. The fuel rods contain a columnof circularly cylindrical pellets of a nuclear fuel arranged in acladding tube. At the bottom of the fuel rods, a bottom plug is arrangedfor insertion into holes provided therefor in the bottom tie plate, andat the top of the fuel rods, a top plug is arranged for insertion intoholes provided therefor in the top tie plate. A small number ofso-called supporting fuel rods retain the fuel bundle and are fixed tothe bottom tie plate and the top tie plate. The fuel bundles or the fuelbundle are/is surrounded by a fuel channel which is normally designedwith a square cross section.

During the burnup of the nuclear fuel, fission gases, which arecontained within the fuel rod, are released. To prevent the pressure onthe cladding from becoming too large, an expansion space is needed forthe fission gases, a so-called fission gas plenum. The fission gasplenum should correspond to 5-10% of the volume of the fuel. For a fuelrod whose diameter is substantially constant and whose active length is4 m, this means that the fission gas plenum should be 0.2-0.4 m. Infull-length fuel rods, the fission gas plenum is arranged in the upperpart thereof, above the reactor core, and hence exerts a minimuminfluence on the efficiency of the reactor. The active length of a fuelrod is the length of that part of the fuel rod which contains fuelpellets and does not comprise a fission gas plenum.

The core is immersed into water which serves both as coolant and asneutron moderator. During operation, part of the water changes intosteam. At the lower part of the fuel assembly, there is only water, butthe higher up in the fuel assembly, the higher the percentage of steam.When the percentage of steam rises, the neutron moderation deterioratessince steam is inferior to water as moderator. The reactivity of thereactor depends on the ratio between fuel and moderator. To improve thereactivity, the water/steam ratio in the upper part of the fuel assemblymust be increased.

One way of obtaining a higher water/fuel ratio in the upper part of thefuel assembly is to replace the majority of the fuel rods with fuel rodswhich are tapering in an axial direction. The General Electric patentapplication EP-0 514 121 A1 discloses a fuel rod which comprises a lowercolumn of fuel pellets with a relatively larger diameter, and an uppercolumn of fuel pellets with a relatively smaller diameter and a claddingtube, which comprises a lower coarser part which surrounds the lowercolumn of fuel pellets, an upper narrower part which surrounds the uppercolumn of fuel pellets, and a transition portion between the lower andupper parts. The fuel rod described in the above EP publication has twofission gas plenums, one in the upper part of the fuel rod and one inthe lower part of the fuel rod.

To obtain an optimum fuel-to-moderator ratio with tapering fuel rods,all or at least the majority of the fuel rods should consist of taperingfuel rods. The disadvantage of tapering fuel rods is that it isexpensive to manufacture fuel pellets with two different diameters andtapering cladding tubes compared with ordinary fuel rods of uniformthickness where all the fuel pellets have the same diameter and thecladding tube is straight. Since the majority of the fuel rods consistof tapering fuel rods, the total cost of the fuel assembly is high.

Another method of reducing the fuel quantity in the upper part of a fuelassembly is to replace some of the fuel rods with part-length fuel rods.Part-length fuel rods have a shorter axial extent than traditionalfull-length fuel rods. Patent document EP-0 336 203 B1 discloses a fuelassembly in which the majority of the fuel rods are full-length fuelrods, that is, they extend from the bottom tie plate to the top tieplate, and a minority of the fuel rods are part-length rods, that is,they extend from the bottom tie plate towards the top tie plate butterminate somewhat below the top tie plate. In order to retain thepart-length rods, they are fixed to the bottom tie plate. The fixing canbe made by providing the bottom plug with threads and screwing it to thebottom tie plate.

Inspection and service of the fuel assemblies are performed at regularintervals, among other things to detect and correct fuel damage as earlyas possible to prevent extensive leakage of fuel and fission products.The inspection and the service take place in a special pool to which thefuel assemblies have been moved. The fuel assemblies are only accessiblefrom above so the fuel rods have to be lifted from above. Thenon-supporting full-length fuel rods can be easily lifted up since theyare not fixed to the bottom tie plate. The fuel rods which are fixed tothe bottom tie plate must be loosened therefrom, which may entailcomplications, especially when the fuel assembly becomes older and thebond may start jamming. A further problem which arises when thepart-length fuel rods are to be lifted up is that it is difficult toreach them and to engage from above. Therefore, specially constructedtools are required to lift out the part-length fuel rods.

The above-mentioned EP application (EP-0 336 203 B1) discloses apart-length fuel rod which has been provided with an extension in theform of a screwed vane (swirl vane) which throws water droplets to thesides so that they hit adjacent fuel rods. In this way, an improveddryout margin is obtained. To attain an increase of the dryout margin,it is sufficient for the swirl vane to extend along the active length ofthe other fuel rods.

To obtain an optimum water/fuel ratio in the fuel assembly, the fuelcolumn in a part-length fuel rod should have a certain optimum height.The optimum height of the fuel column may be obtained by calculations. Aproblem with part-length fuel rods is where to place fission gas plenumsto give a minimum influence on the efficiency of the reactor. Theabove-mentioned EP application (EP-0 336 203 B1) discusses this problemin column 18, lines 52-58, and column 19, lines 5-22. One way is toarrange fission gas plenums in the upper part of the part-length fuelrods, that is, above the optimum height of the fuel column. However,placing fission gas plenums in the upper part of the part-length fuelrod entails several considerable drawbacks. One drawback is that part ofthe volume which could have been filled with a moderator cannot beutilized, and this in a region where moderators are already an in shortsupply. This solution also provides a number of relatively large fuel-and moderator-free volumes centrally in the core, which is not good,either from the neutron point of view, or from the efficiency point ofview. As a better solution to the problem it is proposed that the whole,or at least a part of, the fission gas plenum should be placed at thebottom of the fuel rod. A disadvantage with this solution is that thetotal fuel quantity in the fuel rod decreases by 5-10%, which leads toan undesired reduction in efficiency.

THE SUMMARY OF THE INVENTION

The invention aims to provide a fuel assembly of the kind described inthe introductory part of the description, which

is easy to detach during inspection and service of the fuel assembly,

is relatively inexpensive to manufacture,

has an optimum water/fuel ratio, and

the fission gas plenums of which are arranged so as to exert the leastpossible influence on the efficiency of the reactor.

By making the plenum part sufficiently narrow, it is sufficient with asmall number of fuel rods with a reduced active length to achieve anoptimum water/fuel ratio in the upper part of the fuel assembly. Sincethe majority of the fuel rods are ordinary fuel rods of uniformthickness, the price of the fuel assembly can be kept low.

Because all the fuel rods in the fuel assembly are of the same length,it is easy to lift the fuel rods out of the fuel assembly for inspectionand service. To lift the fuel rods, no special tools or any extra stepare necessary compared with lifting traditional fuel rods.

By extending the fission gas plenum in the longitudinal direction of thefuel rod, a large fuel- and moderator-free volume is prevented frombeing formed centrally in the fuel assembly. Since no fission gas plenumis needed at the bottom of the fuel rod, almost the entire height in thefuel part may be utilized for fuel pellets, which increases theefficiency of the fuel assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail in the following withreference to the accompanying drawings, which show as an example anadvantageous embodiment of a fuel assembly according to the invention.

FIG. 1 schematically shows one embodiment of a fuel assembly accordingto the invention in a section through the line 1--1 in FIG. 2.

FIG. 2 shows the fuel assembly in FIG. 1 in a view perpendicular to ahorizontal plane through the line in FIG. 1.

FIG. 3 schematically shows an embodiment of a fuel rod with a reducedactive length according to the invention.

FIG. 4 schematically shows another embodiment of a fuel rod with areduced active length.

FIG. 5 shows an example of a supporting sleeve.

FIG. 6 shows in more detail part of the fuel assembly according to theinvention in a section through the line 6--6 in FIG. 2.

FIG. 7 shows a section through the line 7--7 in FIG. 2.

FIG. 8a shows a section through the line 3--3 in FIG. 6.

FIG. 8b shows a section through the line 4--4 in FIG. 6.

FIG. 8c shows a section through the line 5--5 in FIG. 6

DETAILED OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a boiling-water fuel assembly 1 which comprises along tubular container with a rectangular cross section, referred to asa cladding tube 2. The cladding tube 2 is open at both ends to form acontinuous flow passage, through which water flows. The cladding tube 2is provided with a hollow support means 3a, 3b of cruciform crosssection, which is secured to the four walls of the fuel channel. Thesupport means consist of four hollow wings 3a and a hollow enlargedcruciform center 3b. The support means 3a, 3b form a vertical channel 4through which non-boiling water flows upwardly through the fuelassembly. The fuel channel with support means surround four verticalchannel-formed parts 5a-5d, so-called sub-channels, with a substantiallysquare cross section. Each sub-channel contains a fuel bundle comprisinga plurality of fuel rods 6a, 6b and 6c arranged in parallel. A fuel rodcomprises a number of cylindrical pellets 7 of uranium dioxide, stackedon top of each other and enclosed in a cladding tube 14a, 14b.

The fuel rods are arranged in a symmetrical 5×5 lattice in which all thefuel rod positions except one are occupied by fuel rods. The fuel rodsare of two different types, namely, fuel rods 6a of uniform thickness,the diameter of which is substantially constant along the whole of itsvertical length, and tapering fuel rods 6b, 6c with a reduced activelength. All the fuel rods in a fuel bundle are retained at the top by atop tie plate 8 and at the bottom by a bottom tie plate 9. The fuel rods6a, 6b, 6c are kept spaced apart from each other by means of spacers 10and are prevented from bending or vibrating when the reactor is inoperation. The spaces between the fuel rods within each sub-channel aretraversed by water and steam. Each fuel bundle comprises two taperingfuel rods with a reduced active length 6b, which are arranged adjacentto each other and close to the cruciform center 3b of the support means,and one tapering fuel rod with a reduced active length 6c which issurrounded by fuel rods 6a of uniform thickness.

FIG. 3 shows a tapering fuel rod with a reduced active length 6b incross section. The fuel rod 6b comprises a fuel part 11, a plenum part12, a transition portion 13 between the fuel part and the plenum part, abottom plug 19a for attachment to the bottom tie plate, and a top plug19b for attachment to the top tie plate. The fuel part 11 is arranged inthe lower part of the fuel rod and comprises a cladding tube 14b whichsurrounds a column of fuel pellets 7. Above the fuel pellets there is ashort space 15 which allows the fuel pellets to expand because ofthermal expansion and irradiation growth.

The plenum part 12 comprises a hollow plenum tube 16, the diameter ofwhich is considerably smaller than the diameter of the cladding tube.The diameter of the plenum tube is preferably smaller than two-thirds ofthe diameter of the cladding tube. The plenum tube is closed at itsupper end. The fission gas plenum 17 consists of the space which issurrounded by the plenum tube 16. The transition portion between theplenum part and the fuel part is open to allow the fission gases formedin the fuel pellets to pass from the fuel part to the fission gasplenum. The lower part of the cladding tube is sealed by a bottom plug18. The plenum part constitutes about one-third of the total length ofthe fuel rod. To increase the strength and the stiffness of the plenumtube, it is suitable for the plenum tube to have a larger thickness thanthe cladding tube.

The spacers 10 are provided with a plurality of openings correspondingto the number of fuel rods 6a and 6c in the fuel bundle. When the fuelrods are passed into or out of the fuel assembly, they are moved throughthese openings. All the spacer holes have the same diameter irrespectiveof whether the thickness of the associated fuel rod is uniform 6a ortapering 6c. FIG. 4 shows a tapering fuel rod 6c with a reduced activelength. It has the same construction as the fuel rod 6b in FIG. 3 andcomprises, in addition, a number of support sleeves 20 arranged on theplenum tube on a level with the spacers. The support sleeves 20 have adiameter corresponding to the diameter of the cladding tube and the taskof these sleeves is to fit into the spacer holes, thus providing supportfor the fuel rod against the spacers. The support sleeves may bedesigned in different ways.

FIG. 5 shows an example of a support sleeve 20, the lower end of whichis provided with inlet holes 21 and the upper end of which is providedwith outlet holes 22, allowing water to flow through the support sleeve.

One problem with a fuel rod with one long and one narrow plenum part isthat it is not as stiff in its upper part as a fuel rod of uniformthickness. This can be solved, for example, with shorter distancesbetween the spacers or with intermediate spacers between the ordinaryspacers. FIGS. 6, 7 and 8a-8c show a solution wherein two adjacenttapering fuel rods are joined together by means of cross bars. The taskof the cross bars is to support and reinforce the fuel rods. They mayalso be designed to direct the water flow and thus reduce the risk ofdryout in the fuel bundle. The cross bars may be welded directly to thefuel rods, or be welded to sleeves which are then fitted over the fuelrods and locked axially.

FIGS. 6, 7 and 8a-8c show three different types of cross bars 25, 27,29. On a level with a spacer 10, a cross bar 25 is arranged whichconnects two tapering fuel rods 6b and at the same time provides supportagainst the spacer 10. In these positions, the spacer 10 has an openingwhich corresponds to two rods so that the rods may be freely pulledupwardly. The cross bar 25 is provided with guiding tabs 26 whichprevent the cross bars from hooking onto the spacers when the fuel rodsare passed out of or into the fuel assembly. The cross bar 27 connectstwo tapering fuel rods. The cross bar 29 connects two tapering fuel rodswhile at the same time providing support against the support member 3b.The cross bars 27 and 29 are provided with deflection vanes 28 whichdeflect the flow of water and hence improve the dryout margin.

To obtain the advantages of the invention, the main part of the plenumpart should have a cross-section area which is smaller than half of thecross-section area in the fuel part. Preferably, the main part of theplenum part should have a cross-section area which is about one-third ofthe cross-section area of the fuel part. By cross-section area it ismeant the total cross-section area, that is, the cross-section areawhich corresponds to the outside diameter of the plenum tube and thecladding tube, respectively. The plenum part should also take up morethan 20% of the total length of the fuel rod to provide theabove-mentioned advantages.

The active length of fuel rods with a reduced active length ispreferably less than 80% of the active length of the other fuel rods inthe fuel assembly. In another embodiment, the plenum part, preferably inits upper part, may comprise a part whose cross-section area is largerthan the cross-section area in the remainder of the plenum part.

I claim:
 1. A fuel assembly for a boiling water reactor comprising abottom tie plate, a top tie plate, a first and a second group ofvertical fuel rods, each comprising a column of fuel pellets surroundedby a cladding tube, wherein the second group of fuel rods has a shorteractive length than the first group of fuel rods, and wherein both thefirst and the second group of fuel rods extend between the bottom tieplate and the top tie plate, the second group of fuel rods comprises afission gas plenum surrounded by a plenum tube arranged above thecladding tube, and the main part of the plenum tube has a cross-sectionarea which is smaller than the cross-section area of the cladding tube.2. A fuel assembly according to claim 1, wherein the main part of theplenum tube has a cross-section area which is smaller than half of thecross-section area of the cladding tube.
 3. A fuel assembly according toclaim 1, wherein the main part of the planum tube has a cross-sectionarea which is smaller than 40% of the cross-section area of the claddingtube.
 4. A fuel assembly according to claim 1 wherein the active lengthof the second group of fuel rods is smaller than 80% of the activelength of the first group of fuel rods.
 5. A fuel assembly according toclaim 1 wherein the fission gas plenum of the second group of fuel rodsconstitutes at least 20% of the total height of the fuel rod.
 6. A fuelassembly according to claim 1 wherein the fission gas planum of thesecond group of fuel rods constitutes at least 30% of the total heightof the fuel rod.
 7. A fuel assembly according to claim 1, furthercomprising a spacer for keeping the fuel rods spaced apart from eachother, and wherein a fuel rod from the second group comprises a supportsleeve arranged on the plenum tube to be in contact with the spacer. 8.A fuel assembly according to claim 1 wherein two adjacently located fuelrods from the second group are joined together by cross bars arranged onthe plenum tube.